The present invention pertains to utilization of shared sources and more particularly to a method and apparatus for controlling the loads of a plurality of units on a shared source such as the electrical loads of a plurality of heating and cooling units.
Generally in the past, control of utilization of a shared source, such as a source of electric power by a plurality of units, has not been centrally provided. What was instead done, in for example the case of cooling units, was that each unit was provided with an individual thermostat and the facility in which the cooling units were housed was supplied with a sufficient electrical supply to handle a simultaneous start-up load of all the units. Recent changes in electrical utility billing procedures penalize this approach by adding a surcharge based on peak power demand during a billing period. The peak demand period measured by the electrical utility is a short period of time, usually fifteen or thirty minutes. A prior method to cut this peak demand has been duty cycling. In duty cycling units are connected and disconnected at a predetermined rate. Duty cycles and connect times of different units can be offset to reduce total peak load. The disadvantage of duty cycling is that it eliminates the flexibility of individual units to respond to changes in conditions. With, for example, an air conditioning system of a restaurant, this inability to meet changes in load, such as the arrival of a large number of customers, severely limits the utility of duty cycling. Problems of this kind are partially overcome by another prior art method in which units are controlled by duty cycles varied in accordance with the previous day's loads of the units upon the source. Any changes in loads or demands from those of the previous day are handled by manually overriding the established duty cycles. This method is effective where loads do not vary much on a daily basis, however, if conditions require the override to be used often, peak load is increased and utility of the method greatly decreased.
It is therefore highly desirable to provide an improved method and apparatus for controlling the loads of a plurality of units on a shared source that duty cycles those units. It is also highly desirable to provide an improved method and apparatus for controlling the loads of a plurality of units on a shared source, that offsets connect times of those units. It is yet also highly desirable to provide an improved method and apparatus for controlling the loads of a plurality of units on a shared source, that can decrease the total connection period of the plurality of units on the source by 20%. It is further highly desirable to provide an improved method and apparatus for controlling the loads of a plurality of units on a shared sources, that can vary duty cycles with variations in load and demand. It is likewise highly desirable to provide an improved method and apparatus for controlling the loads of a plurality of units on a shared source, that has override and underride duty cycle levels for each unit, which may be used in situations of unusual load but which do not eliminate the offset connect times of the units nor duty cycling of the units. It would finally be highly desirable to provide an improved method and apparatus which meet all of the above desired features.